Velocity of ultrasound as an indicator of bubble content

The principle has been evaluated of detecting stationary tissue gas bubbles simply by measuring the velocity of ultrasound. Agar gel has been used to simulate tissue. At wavelengths appreciably larger than bubble diameter (at least 10:1) the ultrasound seems to "view" the gel-gas mix as though it were one medium, when the velocity of pulsed ultrasound is reduced approximately as predicted theoretically from the decreased modulus. The velocity of sound shows an appreciable (10%) decrease for only 0.65% of the gas phase as bubbles of 20-200 microns diameter and drops to about one-third of the bubble-free value for only 0.79% gas by volume--values well in excess of those theoretically predicted to elicit the symptoms of limb bends. These large changes in the velocity of sound can probably be measured with a much less expensive unit than the one used in this study and would seem to warrant further investigation as a very simple method for detecting bubbles in tissue--whether intravascular or extravascular.